Various methods are known for measuring bone characteristics using acoustic techniques to identify patients in need of treatment for bone conditions and diseases. Many acoustic techniques utilize a first transmitting transducer to provide an acoustic signal, typically at ultrasonic frequencies, to the subject from a first external location and a second receiving transducer at a second external location disposed on the opposite side of the bone of interest to receive the signal transmitted by the first transducer through the bone and intervening soft tissue. Typically, such transducers are coupled to the subject through a suitable fluid, such as water or through ultrasound transmission gel.
One acoustic measure of bone often used is the so-called Broadband Ultrasound Attenuation (BUA), typically quoted for the frequency range of approximately 200 to 600 kHz. The BUA is defined as the slope of a linear logarithmic-amplitude versus frequency plot of the energy transmitted through the heel. BUA measures are typically performed by Fourier transforming the signal produced in the receiving transducer due to transmission of a broadband acoustic pulse through the bone undergoing measurement. The Fourier components of the received signal are typically ratioed to the corresponding components measured through a medium of known spectral attenuation characteristics so that the slope of the bone attenuation versus frequency may be derived.
Trabecular bone is known to have the effect of preferentially attenuating higher frequencies--the extent of the preferential attenuation is known to decrease as the bone becomes increasingly porous. Thus, the BUA similarly decreases for more porous bone. BUA measurements are complicated by a variety of factors. For example, the BUA computed may depend not only on the apparatus used, but on the length and portion of the time domain record that is used, the type, if any, of window function used with the data, the frequency range and method used for estimation of the slope, and the methods used for calibration. Further difficulties may result from the presence of received signals that result from transmission through the bone via multiple paths.
The early portion of the received waveform may be more representative of the measured body part. It is desirable, moreover, to express the results of measurements made with respect to early or other transient portions of the received signal in terms of BUA, since it has been common practice to relate BUA values to bone condition empirically.
For clinical utility, measured and quoted characteristics must be highly reproducible from measurement to measurement for a given subject, whether the measurements are made with one, or more than one, measurement unit. In order to monitor the reliability and repeatability of the measurements, standards of various sorts have been provided to simulate the attenuation properties of bone, namely preferential attenuation of higher frequencies. One type of standard requires fabrication of a model heel structure, or phantom, such as an epoxy-resin matrix filled with a fluid, or an epoxy resin filled with particles of another material such as tungsten powder or glass beads. Another type of standard known in the art is an electronic standard that simulates the spectral effect of an attenuating bone.